The present invention relates to a reception system of a wireless communication signal which is spectrum scrambled for improving speech secrecy and/or co-channel interference over a transmission radio channel, in particular, relates to such a reception system for PM modulation (phase modulation) signals. The present invention is effective in improving fading noise reduction.
FIG. 1 is a prior PM receiver which has a privacy unit, and the numeral 1 is a reception antenna, 2 is a PM demodulator, 3 is a spectrum de-scrambler for privacy purpose, 4 is an output terminal, and the symbols (a) and (b) are observation points.
FIGS. 2(a) and 2(b) show spectrum of a signal (upper portion), and a noise (lower portion). When a demodulated signal has noise as shown by the shaded area of FIG. 2(a), that noise spectrum is inverted as shown in FIG. 2(b) by a spectrum de-scrambler (spectrum inverter).
FIG. 3 is a prior PM transmitter used as a transmitter for the receiver of FIG. 1, and the numeral 5 is an input terminal, 6 is a spectrum scrambler, 7 is a PM transmitter, and 8 is a transmission antenna, and the symbols (c) and (d) are observation points.
FIGS. 4(a) and 4(b) show the spectrums at the points (c) and (d) of FIG. 3.
Conventionally, the transmitter of FIG. 3 which has a spectrum scrambler 6 at the front end of the PM modulator 7 is used as a transmitter, and the receiver of FIG. 1 demodulates the PM signal by the PM demodulator 2, and then, demodulated signals are de-scrambled to reproduce original spectrums. The spectrum scrambling feature in the present explanation is, however, restricted to be a simple spectrum inversion.
The transmission radio channel between a transmitter and a receiver is a so-called PM fading channel which affects the transmission signals by fading noise.
Mobile communication is subject to be considered over the fading channels. FIG. 5 shows an average power spectrums of noise through a PM fading channel, where the horizontal axis shows frequency and the vertical axis shows logarithmic amplitude. The curve (a) shows the noise characteristics when the reception level is 10 dB.mu. (the field strength) at the end of the service area in a mobile communication system, and the curve (b) shows the case when the reception level is 22 dB.mu. (the field strength) at the center of the service area. It should be noted in FIG. 5 that the noise with -20 dB/decade of integration characteristics are observed in the whole area. Accordingly, the noise is able to be shown by the shaded triangle as shown in FIG. 2(a).
When a demodulated signal has noise as shown by the shaded area of FIG. 2(a), that noise spectrum is inverted as shown in FIG. 2(b) by the spectrum de-scrambler 3 (spectrum inverter).
When an FM demodulator is used instead of a PM demodulator, a fading noise with flat spectrum as shown in FIG. 6(b) is observed. FIG. 6(a) is the same as FIG. 2(a) which shows the spectrum of the output of a PM demodulator, and FIG. 6(c) is the same as FIG. 2(b) which is the output spectrum of the spectrum scrambler. Those three patterns of the noise characteristics as shown by the shaded area in FIGS. 6(a), 6(b) and 6(c) are the typical noise spectrums.
The table 1 shows the audio level of those three spectrums obtained through both the Zwicker's analysis method and experiments. As shown in the table 1, when the noise powers of the three patterns are set to be equal to each others, the audio level of FIG. 6(a) is the lowest of the three, the spectrum of FIG. 6(b) is higher than that of FIG. 6(a) by about 4 dB in audio level, and the audio level of FIG. 6(c) is the highest and is higher than about 10 dB compared with that of FIG. 6(a).
Accordingly, when the receiver of FIG. 1 is used as a PM demodulator, the S/N is worse by about 10 dB in comparison with the S/N when no spectrum inversion is used.
TABLE 1 ______________________________________ (Audio level of typical spectrums) Spectrum pattern FIG. 6(a) FIG. 6(b) FIG. 6(c) ______________________________________ Anal- Zwicker method 30.1 33.9 40.6 ysis (Sone) Relative 0 3.5 10.5 indication (dB) Exper- CCIR (dB) -29 -26 -21 iments DIN (noise) (dB) -30 -27 -23 ______________________________________